On a DL flight out of Salt Lake City a few years ago we started hearing regular thumps (like a bowling ball rolling around in the trunk of your car) right after takeoff. Then we noticed that we weren't climbing as fast as usual. Then we noticed that we were circling back. Then we noticed that we were landing. By the time we figured out that we ought to be scared, we didn't have to be any longer.

Turned out that someone hadn't secured the baggage compartment door properly. It opened during flight and cut off airflow into the (rear-mounted) engine on that side, which flamed out. This took away enough pull on the door for it to move away from the engine intake. The automatic restart gizmos worked, after which the engine inflow pulled the door right onto the intake again. And so it went, until the pilot shut down that engine and landed with the other one.

Needless to say, Delta was very nice about reaccommodating everyone.